This invention relates generally to deflecting yoke assembly for a picture tube, and more particularly the present invention relates to a deflecting yoke assembly for making a trapezoidal raster required in a picture tube used for projecting TV pictures on a large screen.
Various types of TV picture projection systems have been developed hitherto so that TV pictures can be projected on a large screen. In some single-picture-tube projection systems, the shape of the raster made in the picture tube is trapezoidal so that a rectangular picture can be obtained on the projection screen. As is well known, such a trapezoidal raster can be obtained by changing the amplitude of the vertical deflection currents applied to the deflecting yoke. Namely, the amplitude of the vertical deflection currents is varied so that it becomes larger and larger in a direction from the beginning of the vertical scanning corresponding to the top of the screen toward the end of the same corresponding to the bottom of the screen. In conventional TV picture projection systems, therefore, a particular circuit arrangement having a reactor has been employed to achieve the above-mentioned change in the amplitude of the vertical deflection currents. As will be described later, such a particular circuit arrangement requires a number of parts, and therefore, it results in increase in manufacturing cost and in assembling time. Furthermore, since the reactor occupies a relatively large space, the entire system tends to be bulky. Moreover, the reactor is constructed of a closed magnetic path having a short gap length, the magnetic characteristic of the core is apt to change due to temperature variation. This lowers the reliability, while the operation of the system is unstable.